There is a great demand for zoom lenses with wider projection angles and greater brightness in projection systems, such as projection televisions using liquid crystal display elements or digital micromirror devices (DMD). An optical system that is telecentric on the reducing side is used in prior art projection systems that employ an X-shaped dichroic prism or a totally reflecting prism on the reducing side. However, the use of a telecentric optical system makes it difficult to correct distortion in zoom projection lenses.
Japanese Laid Open Patent Applications H10-268193 and 2000-292701 disclose improved zoom lenses, each of which uses a positive lens component that is outermost on the enlarging side of the zoom lens in order to reduce distortion. However, having a positive lens component as the outermost lens component on the enlarging side requires a larger lens diameter in order to maintain image brightness and a wide projection angle, which reduces the desired compactness of the zoom lens.
Additionally, it is known that an aspherical lens surface is useful for correcting aberrations, including distortion. An aspherical lens surface can be obtained by processing a glass lens or a plastic lens so as to form an aspherical surface on it. However, obtaining an aspherical lens surface on a glass lens is a costly process, especially for a larger diameter lens. Moreover, in order to prevent distortion, aspherical surfaces are required on the outermost surface of both the enlarging side and the reducing side of projection zoom lenses. Further, when an aspherical plastic lens element is used, the optical properties of the lens element fluctuate significantly with temperature. When a plastic lens element is used that has a large refractive index, it is especially difficult to design a compact projection lens with small temperature-dependent fluctuations in optical properties. Aberrations, including distortion, are usually better corrected if plastic lens elements that are used do not have a large refractive index. However, in such a case, it is difficult to produce a zoom projection lens that is compact.
The present invention is a zoom lens useful for image pick-up devices, such as CCDs and camera tubes, film cameras, and particularly for use as a projection lens for projection televisions that use digital image devices, such as digital micro devices (DMDs). The present invention provides a bright, compact, and inexpensive zoom lens having less temperature dependent fluctuations in optical properties as well as reduced aberrations, especially distortion.